It is known in the art to seal off the vent openings in the pump barrel of a so-called "up-position shipper" or "uplocking pump". Since the vent openings are located adjacent the upper end of the barrel, it is a relatively uncomplicated matter for the piston to simply cover the vent openings when the plunger is fully extended and locked in that position. By covering the vent openings in this way, the container may be laid on its side without product entering the barrel through the vent openings and then leaking out the pump around the plunger or otherwise.
But in down-locking pumps the piston is at the lower, opposite end of the barrel when the plunger is in a locked position. Consequently, the piston cannot be used to seal off the vent openings. Thus, then, provides an avenue for product to enter the barrel above the piston and find its way to a leakage path along the plunger or otherwise. Furthermore, product may simply accumulate above the piston so that, on the first upstroke of the plunger after unlocking, the product may be forced up out of the pump along the plunger, producing an unsightly mess.